A WIND-TUNNEL STUDY OF THE SALTATION OF HETEROGENEOUS (SIZE, DENSITY) SANDS

Results of wind-tunnel experiments designed to investigate selective sorting of sands by size (d) and density ((sigma)) document the composition and texture of the saltating population entrained from a source sand with non-uniform size and mineralogy. In the source sand quartz ((sigma) = 2.65 g/cm('3)), garnet ((sigma) = 4.3 g/cm('3)) and olivine ((sigma) = 3.35 g/cm('3)) had the same size range (0.08 - 0.5 mm), same mean size (0.2 mm), and similar size-frequency distributions. Therefore, sorting in experiments was not confounded by a lack of availability of coarse heavies.; In four experiments, at U(,*) values of 20,40,52 and 110 cm/sec, paired mean sizes of quartz and garnet in saltation are not statistically significantly different at each height from 1 to 5 cm above the bed. Shapes of size-frequency distributions near the bed (y < 2 cm) reflect initial distributions for both minerals. With increasing U(,*), size distributions at y < 2 cm have larger (')d and greater coarse tails. At any U(,*) samples higher in the saltation cloud (y (GREATERTHEQ) 2 cm) have more peaked distributions with smaller (')d, smaller maximum size and insignificant coarse tails. Distributions of samples at y (GREATERTHEQ) 2 cm change little with increasing U(,*). Existing formulae for entrainment do not explain the observed size relationships. The observation of equal mean sizes of quartz and garnet in saltation when equal sizes are available, suggests that small heavy-to-light mineral size ratios measured in aeolian sands may reflect source restrictions on heavies more than they indicate sorting by entrainment and transport.; Integrated mass fluxes of both quartz and garnet increase with increasing U(,*), but mass flux of garnet is significantly less than that of quartz in all four experiments. Relative proportions of quartz and garnet in saltation differ from their proportions in the stock sand, so transport rates and entrainment rates are not simple functions of availability. Calculated grain velocities are greater in general for finer or for less dense grains; velocities increase with increasing wind velocity even though trajectory heights change little. Calculated entrainment rates are strong functions of both d and (sigma). The data suggest that the composition of the saltating population results principally from differential entrainment rates and to a lesser extent from differential transport rates. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI...